Diabetes mellitus (DM) is a metabolic disease that is characterized by absolute or relative deficiency of insulin. Relative or absolute deficiency of insulin can lead to hyperglycemia, which further cause metabolism disorder of three major nutrients, and ultimately affect normal physiological functions of the patients and cause complications. The patients with diabetes worldwide is increasing day by day, among adults aged 20-79 years, the number of the patients with diabetes in 2013 has reached 382 million, and it is estimated to reach 439 million by 2030. Among them, the population with diabetes in China is the highest in the world, and is about 110 million at present and still growing fast. The rise in obesity drives an increase in diabetes, and approximately 90% of people with type II diabetes may be classified obese. There are 246 million people worldwide with diabetes, and by 2025 it is estimated that 380 million will have diabetes. Many have additional cardiovascular risk factors including high/aberrant LDL and triglycerides and low HDL. Diabetes cannot be cured at present, and the patient can only rely on drugs for life. Traditional anti-diabetic drugs have their shortcomings, and there is an urgent need to develop new anti-diabetic drugs.
Compared with small-molecule chemical drugs and macro-molecule protein drugs, peptide drugs have their own advantages: first, most of them are derived from endogenous peptides or other natural peptides, clear structure, and have clear structures and mechanisms of action; second, compared with general small-molecule drugs, they have higher activity, less dosage, less toxic side effects, with amino acids as the end product of metabolism (free of toxic side effects); third, compared with the foreign proteins, they have low immunogenicity, and can be chemically synthesized, and the product has high purity and controllability on quality; and fourth, polypeptide drugs are often able to avoid the gastrointestinal digestion and overcome the drawbacks that protein molecules are destroyed by digestive enzymes and thus cannot be orally administrated.
Glucagon-like peptide (GLP-1) belongs to the family of incretins, which is a polypeptide mainly secreted by intestinal mucosal L cells, and has two active forms: GLP-1-(7-37) and GLP-1-(7-36)-amide. GLP-1 play an antidiabetic role by combining with the specific receptor glucagon like peptide-1 receptor (GLP1R), the main physiological functions are: to improve islet βcell function, to promote the secretion of insulin, to reduce postprandial blood glucose and maintain blood glucose homeostasis, to increase insulin biosynthesis, to inhibit glucagon secretion, to inhibit gastrointestinal motility especially gastric emptying so as to increase satiety, to decrease appetite and control body weight. Compared with traditional antidiabetic drugs, the advantage of the GLP-I is to maintain glucose homeostasis and control body weight effectively. (Cardiovascular effects of glucagonlike peptide-1 agonists, Am J Cardiol. 2011; 108:33B-41B. Cardiovascular effects of the DPP-4 inhibitors, Diab Vasc Dis Res. 2012; 9:109-16. Incretin mimetics as a novel therapeutic option for hepatic steatosis, Liver Int. 2006; 26: 1015-7.). GLP-1 has a very short half-life, and is degraded by dipeptidyl-peptidase-IV (DPP_IV) or Neutral endopeptidase (NEP) 24.11 soon after being secreted. Therefore, GLP-1 cannot be directly used in the clinic, and there is a need to develop an enzymolysis-resistant GLP-1 receptor agonist.
However, clinical trials have demonstrated the shortcomings of GLP-1 receptor agonists are also obvious, mainly in the following aspects:
However, clinical trials have proved that GLP-1 receptor agonist also has very obvious shortcomings, mainly in the following aspects: first, the short half-life leads to intensive injection frequency, and brings inconvenience to the patients; second, pharmacokinetics and security are not clear, and it is unclear how the introduced foreign chemical groups are metabolized and excreted and how do they influence the human body, and thus further investigation is needed. The latest preclinical study show that compared to a pure GLP-1R agonist, balanced glucagon like peptide-1 receptor and glucagon receptor (GLP-1R/GCGR) dual target agonist exhibits a more effective, safer treatment effect on obese mice, and also improve the blood glucose control (A new glucagon and GLP-1 co-agonist eliminates obesity in rodents, Nat Chem Biol. 2009; 5:749-57.). Related to this is the study on oxyntomodulin (OXM). OXM is a short peptide hormone secreted by intestinal epithelial L-cells, which is composed of 37 amino acids, and is an endogenous precursor of glucagon. OXM is a balanced GLP-1R/GCGR dual target agonist, its potency on glucagon receptor is 2-fold lower than on GLP-1 receptor, and is lower than that of the natural glucagon and GLP-1 on their respective receptor. Although the biological activity of natural OXM is low, the clinical research shows that consecutive subcutaneous injection of natural OXM over a four-week period still can significantly reduce the patient's body weight and decrease food intake. (Subcutaneous oxyntomodulin reduces body weight in overweight and obese subjects: a double-blind, randomized, controlled trial, Diabetes. 2005; 54: 2390-5.) OXM has a very short half-life, which may be inactivated rapidly by dipeptidyl peptidase IV (DPP-IV) on cell surface, and has poor stability in vivo.
GLP-1R/GCGR co-agonist has been proved to significantly reduce body weight and fat content in high fat diet-induced obesity rats (DIO rat), which is superior to any pure GLP-1R agonist, and also improve the blood glucose control. These changes may be associated with reduction in food intake and substantial increase in energy consumption. Another report pointed out that a long-acting GLP-1R/GCGR dual target agonist having protease resistance, compared with the long-acting GLP-IR agonist having the same effect, can significantly reduce body weight, reduce triglycerides and resist hyperglycemia. In addition, the long-acting GLP-1R/GCGR dual target agonist may improve metabolic parameters, such as blood insulin, leptin and adiponectin (Unimolecular dual incretins maximize metabolic benefits in rodents, monkeys, and humans, Sci Transl Med. 2013; 5:209.).
In summary, the development of GLP-1R/GCGR dual target agonist is currently the main direction for development of polypeptide drugs for treating diabetes. In recent years, researchers have developed several potential OXM analogues for injecting once a day and once a week. Among them, the most widely used modifier is mono-methoxy polyethylene glycol (methoxypoly ethylene glycol, mPEG), by increasing the molecular exclusion volume of OXM, reducing renal filtration clearance rate of the drug molecule, thereby prolonging the mPEG-modified drug's plasma half-life, so as to achieve the goal of weekly injection. Although this method can significantly improve the half-life of polypeptide drugs, the biological activities of most of the proteins are decreased with different degrees. Even more dangerous is, mPEG is a molecule that cannot be metabolized in human body, the polypeptide protein drugs derived from it may lead to renal vacuolation (Short communication: renal tubular vacuolation in animals treated with polyethylene-glycol-conjugated proteins, Toxicol Sci. 1998; 42: 152-7; Safety assessment on polyethylene glycols (PEGs) and their derivatives as used in cosmetic products. Toxicology, 2005; 214:1-38.). The vast majority of drug molecules modified by mPEG is used for tumor treatment, thus the toxicity of PEG is often overlooked greatly. From the nature of chemical point of view, the biochemical and physiological properties of polypeptide drugs may be solved fundamentally only by optimizing the polypeptide sequence, which is the fact that is recognized by biochemists; and on this basis, various indicators such as activity and stability of polypeptide drugs may be further improved by biochemical means.